The present invention relates to a supercritical air-foil, particularly a wing profile for an aircraft, or a related type air-foil having a specific design Mach number not exceeding about 0.85 and having a maximum thickness, in units of chord length of d/l=0.13.+-.20%.
Supercritical wing profiles are known in a variety of profile contours and shapes resulting accordingly in different pressure distributions along upper and lower surfaces of the air-foil. Basically, two lines of development have resulted here, one of which is represented by German printed patent application No. 2,254,888 (and also U.S. Pat. No. 3,952,971). The profiles as disclosed in this patent develop approximately constant pressure along the upper surface because the supersonic speed zone along the upper surface covers a rather large portion of the profile depth. These profiles have also be termed rooftop profiles; they do indeed produce an increase in lift but they have the disadvantage of an increasing drag resistance of the profile due to the rather abrupt pressure increase in more rearwardly located areas of the profile. Moreover, it was found that these particular profiles, being optimized for a particular design case, exhibit significant disadvantages and drawbacks for off-design operating conditions. For example, these profiles have the tendency that for lower than the design Mach number two shocks are produced which increase the drag resistance significantly. On the other hand, in the case of an increase in the Mach number, at which the profile is operated, the supersonic speed zone is bounded in the rear portion of the profile by shock which becomes rapidly stronger with an increases in speed and leads directly to an increase in resistance and to an increase in the pressure gradient, which in turn endangers increasingly the boundary layers as far as its separation is concerned due to the shockwave that is produced.
The second line of development is particularly derivable from the two German printed patent applications Nos. 2,608,414 and 2,626,279 (see also U.S. Pat. No. 4,072,282). The profiles disclosed in these patents are also known as the Peaky profiles. The pressure distribution for these profiles is particularly characterized by a suction peak in the nose region and have, therefore, rather favorable resistance values for the design parameters. On the other hand, this particular type of profile has a drawback in that the pressure distribution is rather sensitive to changes in the angle of attack as well as to changes in the Mach number. The rear boundary of the supersonic zone sonic line terminates in these off-design operating cases in a rather rapidly increasing shock at shifting positions. The minimum lift as produced is prematurely limited in that the rather rapid increase in resistance introduces a non-linear lift moment which is the result of a shock-induced boundary layer separation. Therefore, an aircraft having such a wing experiences significant changes in pitching moment if the speed deviates significantly from the design speed and/or if the angle of attack deviates from the design value. Accordingly, the manouverability of such a craft suffers due to the sensitivity of the flight parameters to these external interferences and changes in operating conditions. For lower speeds the Peaky profile is endangered as far as lift is concerned by laminar nose separation.